GaN, AlN, and InN which are nitride semiconductors or materials containing a mixed crystal thereof have a wide band gap and have been used for high-power electron devices, short wavelength light emitting devices, or the like. Among the above, as the high-power electron devices, a technique relating to a field effect transistor (FET), particularly, a high electron mobility transistor (HEMT) has been developed. The HEMT employing such nitride semiconductors may realize large current, high voltage, and low on-state resistance operation, and therefore has been used for high-power and high-efficiency amplifiers, high-power switching devices, and the like.
For example, GaN which is a nitride semiconductor has a band gap of 3.4 eV which is larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs and has high breakdown electric field strength. Therefore, GaN is very promising as a material of a semiconductor device for power source with which high-voltage operation and high power are obtained.
Examples of the HEMT described above include a HEMT employing GaN as an electron transit layer and AlGaN as an electron supply layer. In the HEMT employing GaN as an electron transit layer and AlGaN as an electron supply layer, distortion resulting from difference between the grating constant of GaN and the grating constant of AlGaN generates in AlGaN. Due to difference between the piezoelectric polarization and the spontaneous polarization of AlGaN thus generated, a high-concentration two dimensional electron gas (2DEG) is generated in the electron transit layer. Therefore, the HEMT has been expected as a high-efficiency switch element and a high withstand voltage power device for electric vehicles and the like. In recent years, in order to reduce the substrate cost, a technique of growing a HEMT formed of a nitride semiconductor on a low-cost Si substrate has been researched.
However, in the case where the HEMT is formed by growing an electron transit layer and an electron supply layer with nitride semiconductors on a substrate, such as a Si substrate, when the substrate has conductivity, a strong electric field is applied to the nitride semiconductor layers. In this case, the strongest electric field is applied particularly to end portions of a drain electrode due to large potential displacement, and thus breakdown or the like is likely to occur at the drain electrode ends, which results in reduction in reliability.
Therefore, in the semiconductor device including nitride semiconductors formed on a substrate such as a Si substrate, a highly-reliable semiconductor device in which breakdown or the like does not occur at the drain electrode ends has been desired.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2002-359256 and
[Document 2] Japanese Laid-open Patent Publication No. 10-32349.